In the course of the Bayer process, aluminium hydroxide is precipitated from a process liquor by controlling process conditions associated with the liquor The precipitate settles at the bottom of a vessel referred to as a thickener and the process of causing precipitates to settle out of a liquor is referred to as thickening. The precipitates will be in the form of particles having a range of sizes, including some coarser and some finer sizes. The proportion of particles in the coarser and finer ranges of size depends on the process conditions.
Thickening results in liquor toward the bottom of the thickener being high in coarser precipitate content, and very viscous, and the liquor near the top of the vessel being very low in finer precipitate content. The process liquor may pass through a series of thickeners; some of which have the effect of separating coarse precipitates from fine precipitates and others substantially removing fine precipitates from the process liquor to create a so-called “clear liquor”. The clear liquor is removed and subjected to further processing steps before being returned to the Bayer process as a caustic liquor used in digestion of bauxite.
Thickeners used to separate the coarse precipitate from the fine precipitate are referred to as classifiers. It will be appreciated that the term “thickener” as used herein after includes a reference to a classifier.
The viscous liquor (also termed a “slurry”) is typically removed from the bottom of the thickener by pumping. However, the high viscosity of the slurry may cause a preferential flow path in the vicinity of the outlet of the thickener for the slurry. As a result, the solids outside the flow path settle and build-up within the thickener. The formed preferential flow path is termed a “rat-hole”.
The effect of rat-holing is that the settled precipitate reduces the overall operational volume within the thickener. This means that process liquor has a shorter residence time in the thickener and, therefore, the viscous slurry extracted from the thickener has a precipitate content that is lower than desired for subsequent processing. This also causes an increase in the precipitate content and precipitate size in process liquor extract from near the top of the thickener, thus affecting negatively the performance of the subsequent processing steps.
Thickeners must, therefore, be shut-down and the settled precipitate cleaned out every two months on average in order to maintain suitable precipitate content in the extracted slurry.
Occasionally, settled precipitate will dislodge and fall into the preferential flow path so that the extracted slurry will have random spikes in precipitate content. This makes downstream processing difficult and necessitates additional control steps to ensure that precipitate content of the extracted slurry is reasonably consistent.
There is a need, therefore, to reduce “rat holes” and build-up of settled solids in thickeners. It will be appreciated that the term “solids” used throughout this specification includes precipitates.